How can I use the Corner pin node interactively?
You can use the Movie Clip Editor 'tracking tools' to provide the vectors.


Open an MCE window and import your clip. Check the dimensions in the
properties panel by pressing the N-key. You should see the image's
Dim x and Dim y value as "Size"
Add four trackers ctrl-click from top left clockwise to bottom left.
Open the Compositor and add to your image a Corner Pin node.
Then add a Track Position node / Map Range node (x2) / Combine RGB
node.
Set the Map range node for the track position x value to Dim x (from
the tracker properties panel), with 0 as the minimum.
Set the Map range node for the track position y value to Dim y, with
0 as the minimum.
Send the Map range node 'results for x' to the RED input of the
Combine RGB node
Send the Map range node 'results for y' to the GREEN input of the
Combine RGB node


Repeat for all four corners, or duplicate the group of 4 nodes 4 times.